https://github.com/rosefromthedead updated 
https://github.com/llvm/llvm-project/pull/186770

>From 62a68eb464df19c17603f99d29a4e5f22ff911e8 Mon Sep 17 00:00:00 2001
From: Rose Hudson <[email protected]>
Date: Wed, 25 Mar 2026 10:15:01 +0000
Subject: [PATCH] [clang][Lex] add -Wnonportable-include-path-separator

Emit an warning when #include paths contain backslashes, with a fixit to
convert them all to '/'. This can help users that build only on Windows
to automatically make their #includes more portable. The warning is off
by default due to being noisy and not always desirable.
---
 .../include/clang/Basic/DiagnosticLexKinds.td |  3 +++
 clang/include/clang/Basic/SourceManager.h     |  9 +++++++++
 clang/lib/Lex/PPDirectives.cpp                | 11 +++++++++++
 clang/test/Lexer/backslash-include-win.c      | 19 +++++++++++++++++++
 4 files changed, 42 insertions(+)
 create mode 100644 clang/test/Lexer/backslash-include-win.c

diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td 
b/clang/include/clang/Basic/DiagnosticLexKinds.td
index 5eceeced311f2..3525cc91d6f61 100644
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -380,6 +380,9 @@ def pp_nonportable_path : NonportablePath,
   InGroup<DiagGroup<"nonportable-include-path">>;
 def pp_nonportable_system_path : NonportablePath, DefaultIgnore,
   InGroup<DiagGroup<"nonportable-system-include-path">>;
+def pp_nonportable_path_separator : Warning<
+  "non-portable path to file '%0'; specified path contains backslashes">,
+  DefaultIgnore, InGroup<DiagGroup<"nonportable-include-path-separator">>;
 
 def pp_pragma_once_in_main_file : Warning<"#pragma once in main file">,
   InGroup<DiagGroup<"pragma-once-outside-header">>;
diff --git a/clang/include/clang/Basic/SourceManager.h 
b/clang/include/clang/Basic/SourceManager.h
index bc9e97863556d..4217b8683da1e 100644
--- a/clang/include/clang/Basic/SourceManager.h
+++ b/clang/include/clang/Basic/SourceManager.h
@@ -1526,6 +1526,15 @@ class SourceManager : public 
RefCountedBase<SourceManager> {
     return Filename == "<scratch space>";
   }
 
+  /// Returns whether \p Loc is located in a <module-includes> file.
+  bool isWrittenInModuleIncludes(SourceLocation Loc) const {
+    PresumedLoc Presumed = getPresumedLoc(Loc);
+    if (Presumed.isInvalid())
+      return false;
+    StringRef Filename(Presumed.getFilename());
+    return Filename == "<module-includes>";
+  }
+
   /// Returns whether \p Loc is located in a built-in or command line source.
   bool isInPredefinedFile(SourceLocation Loc) const {
     PresumedLoc Presumed = getPresumedLoc(Loc);
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index 4a854c213926b..ebf7f6f6da606 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -2728,6 +2728,17 @@ Preprocessor::ImportAction 
Preprocessor::HandleHeaderIncludeOrImport(
       Diag(FilenameTok, DiagId) << Path <<
         FixItHint::CreateReplacement(FilenameRange, Path);
     }
+
+    bool SuppressBackslashDiag =
+        FilenameLoc.isMacroID() ||
+        SourceMgr.isWrittenInBuiltinFile(FilenameLoc) ||
+        SourceMgr.isWrittenInModuleIncludes(FilenameLoc);
+    if (!SuppressBackslashDiag && OriginalFilename.contains('\\')) {
+      std::string SuggestedPath = OriginalFilename.str();
+      std::replace(SuggestedPath.begin(), SuggestedPath.end(), '\\', '/');
+      Diag(FilenameTok, diag::pp_nonportable_path_separator)
+          << Name << FixItHint::CreateReplacement(FilenameRange, 
SuggestedPath);
+    }
   }
 
   switch (Action) {
diff --git a/clang/test/Lexer/backslash-include-win.c 
b/clang/test/Lexer/backslash-include-win.c
new file mode 100644
index 0000000000000..31e282c48e69c
--- /dev/null
+++ b/clang/test/Lexer/backslash-include-win.c
@@ -0,0 +1,19 @@
+// REQUIRES: system-windows
+// RUN: mkdir -p %t/backslash
+// RUN: cp %S/Inputs/case-insensitive-include.h 
%t/backslash/case-insensitive-include.h
+// RUN: %clang_cc1 -fsyntax-only -Wnonportable-include-path-separator -I%t %s 
2>&1 | FileCheck %s
+
+#include "backslash\case-insensitive-include.h"
+// CHECK: non-portable path to file
+// CHECK: specified path contains backslashes
+// CHECK: "backslash/case-insensitive-include.h"
+
+// Despite fixing the same span, nonportable-include-path is still a separate 
diagnostic
+// that can fire at the same time.
+#include "backslash\CASE-insensitive-include.h"
+// CHECK: non-portable path to file
+// CHECK: specified path differs in case from file name on disk
+// CHECK: "backslash\case-insensitive-include.h"
+// CHECK: non-portable path to file
+// CHECK: specified path contains backslashes
+// CHECK: "backslash/CASE-insensitive-include.h"

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